Painting wont hold as Sophie has stated but you can stain travertine. You will need to strip the sealer from the stone first. What sealer did you use to seal the stone? If it's a penetrating sealer you can use KlenzAll by Stone Tech to remove the sealer. A test area is recommended to help determine the amount of times will be needed to wash the area free of the penetrating sealer. You will want to use the heavy duty dilution when mixing KlenzAll. This process can also remove enhancing sealers but complete removal of the enhancing sealer is not always possible.
Once the sealer has been removed you can stain the travertine. Couple years ago a customer of mine whitewashed their travertine and it turned out spectacular. Wish I had a photo...
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